Designating or pertaining to a terrestrial division consisting of the northern and mountainous parts of both the Old and the New World; -- equivalent to the Holarctic region exclusive of the Transition, Sonoran, and corresponding areas. The term is used by American authors and applied by them chiefly to the Nearctic subregion. The Boreal region includes approximately all of North and Central America in which the mean temperature of the hottest season does not exceed 18° C. (= 64.4° F.). Its subdivisions are the Arctic zone and Boreal zone, the latter including the area between the Arctic and Transition zones.